Use structured suggestion for closure needed instead of inner function

[estebank]

We currently emit the following error

error[E0434]: can't capture dynamic environment in a fn item
  --> $DIR/bad-env-capture3.rs:4:31
   |
LL |         fn bar() { log(debug, x); }
   |                               ^
   |
   = help: use the `|| { ... }` closure form instead

It'd be nice if we provided a structured suggestion instead.

[XiangQingW]

@estebank Excuse me, do you mean that we should use the span_suggestion method of DiagnosticBuilder to make a suggestion?

[estebank]

Correct. The tricky part will be finding the appropriate spans to suggest with. It seems to me we could use a multipart suggestion and replace the leading fn bar( with let bar = | and the ) with |.

This would get trickier if there are type parameters because you'd have to probably just remove the type of the affected argument, but we can fallback to the current note in cases like that.

[XiangQingW]

I'm trying to carry out the idea and the span of fn bar can be got from the inputs of visit_fn.

fn visit_fn(&mut self, fn_kind: FnKind<'tcx>, declaration: &'tcx FnDecl, sp: Span, _: NodeId)

However, I cannot find a way to get the span of ( and ). Could you please give me some advice if you have free time.

[estebank]

Triage: no change